The present invention relates generally to a spiral-mode or sinuous microstrip antenna with variable ground plane spacing.
A new class of microstrip antenna has been reported in various journal articles. Most of the reported journal articles are due to research sponsored/supported by the Wright laboratory at Wright-Patterson Air Force Base, Ohio. This microstrip antenna eliminates the cavity used typically in spiral and sinuous type antennas. A small spacer between the antenna element and ground plane replaces the cavity. This significantly reduces the depth of the antenna and permits antenna shapes which conform to the vehicle skin. The microstrip has demonstrated higher gain and efficiency than the cavity, but only over a limited bandwidth less than that of existing cavity backed antennas. In existing microstrip antennas the spacer between the ground plane and antenna element is a constant depth or thickness. The following articles are of interest with respect to such microstrip antennas.
1. J. J. H. Wang and V. K. Tripp, "Design of Broadband, Conformal, Spiral Microstrip Antenna", 1990 URSI Radio Science Meeting, Dallas, Tex., May 1990.
2. J. J. H. Wang and V. K. Tripp, "Design of Multioctave Spiral-Mode Microstrip Antennas", IEEE Trans. Ant. Prop., Mar. 1991.
3. V. K. Tripp and J. J. H. Wang, "Multi-octave Microstrip Antennas", Proceedings of the 1990 Antenna Applications Symposium, Allerton Park, Ill., September 1990.
The following United States patents are of general interest with respect to microstrip and cavity backed antennas.
5,008,681--Cavallaro et al PA1 5,006,858--Shirosaka PA1 5,003,318--Berneking et al PA1 4,990,927--Ieda et al PA1 4,573,212--Lipsky PA1 4,658,262--DuHamel
The patents to Cavallaro et al, Shirosaka, and Ieda et al deal with the mounting of microstrip antennas. The patent to Berneking et al discloses a wide bandwidth. The patent to Lipsky discloses a typical cavity backed antenna, in a device having a detector mixer unit integrated with a spiral antenna. The patent to DuHamel discloses a cavity backed sinuous antenna.
The most prevalent receiving antenna on military aircraft is the cavity backed and loaded spiral. Recently, the similar cavity backed and loaded sinuous has begun to supplement the spiral (See James P. Scherer, "The Dual Polarized Sinuous Antenna", Journal of Electronic Defense, Aug., 1990, hereby incorporated by reference; and the DuHamel U.S. Pat. No. 4,658,262, also hereby incorporated by reference). These cavity loaded antennas are widely deployed because of their wide band (900%) frequency operation. A cross section of a typical cavity spiral is illustrated in FIG. 1. It consists of an antenna element such as copper over a cavity containing materials which absorb the field currents and prevent reflections back to the antenna element. If the absorbing material is not employed, the cavity antenna is capable of high efficiency; however, in this seldom used form, the bandwidth is only about 10% which severely limits its application. The use of absorbing material reduces the gain by 3 dB and efficiency to less than 50% but permits operation over very large bandwidth. The reduction of efficiency severely limits its application for transmitting. The cavity complicates the installation of the antenna on aircraft and also prevents conformal installation. The size of the cavity and the non conformal installation required become very important for frequencies below 2 GHz. As a result the cavity loaded antenna has found its greatest application for receive functions above 2 GHz. The absorbing material varies according to the application, but can be a low dielectric material such as foamed polystyrene loaded with graphite for laboratory experimentation.
The sinuous form of the antenna of FIG. 1 is shown in FIGS. 7, 8 and 9; which are copies of FIGS. 1, 2 and 3 of the DuHamel patent.
An antenna in accordance with an embodiment of the prior art DuHamel invention is shown in FIG. 7 with the spherical coordinate system r, .theta., .phi.. It consists of four sinuous arms 11 lying on a plane and emanating from a central point 12 located near the Z axis. The arms interleaf each other without touching and are defined such that a rotation of the antenna of 90.degree. about the Z axis leaves the antenna unchanged. The arms are excited by a feed network and a four wire transmission line (not shown) connected to the arm at the inner-most points 12 so as to produce currents with equal magnitudes and a progressive phase shift of +90.degree. or -90.degree. to achieve two senses of circular polarization (CP). The antenna is placed over a conducting cavity 13 (usually filled with absorbing material) so as to produce a rotationally symmetric unidirectional pattern with the peak on the Z axis. The feed network, which may consist of two baluns and a 3 db 90.degree. hybrid, may be placed underneath the cavity. The four wire transmission line runs from the bottom of the cavity along the Z axis to the feed points 12. Without the cavity, the antenna produces a rotationally symmetric bi-directional pattern with opposite senses of CP in opposite directions. The arms consist of metal strips 14 with width which increase with distance from the center. Printed circuit board techniques may be used to obtain strips with a width and thickness of a few thousandths of an inch.
The sides of the sinuous arms are defined by curves related to the curve 16 shown in FIG. 8. In general the curve consists of P cells numbered 1 to P. The line ABC forms cell number 1, the line CDE forms cell number 2, and so on. The radii R.sub.p define the outer radius of each cell. The design parameters .alpha..sub.p, a positive number and .tau..sub.p, a positive number less than 1, define the angular width and ratio of inside to outside radius for each cell respectively. The equation for the curve p.sup.th cell is given by ##EQU1## where r and .phi. are the polar coordinates of the curve.
The radii r.sub.p are related by EQU R.sub.p =.tau..sub.p-1 R.sub.p-1
This type of cell is termed a sine-log cell. If .alpha..sub.p and .tau..sub.p are independent of p, then the curve is a log-periodic of the logorithm or the radius r. If .alpha..sub.p and .tau..sub.p are not independent of p then we may refer to the curve as a quasi-log periodic curve or a tapered alpha and tau curve.
If we define .tau..sub.p by ##EQU2## then the radial lengths of the cells are identical. If, in addition, .alpha..sub.p is independent of p, the curve is a periodic function of the radius. In this case the curve may be defined by EQU .phi.=.alpha. sin(180 P r)
where P is the number of cells for a normalized radius of R.sub.1 =1. These cells are termed sine cells. For .tau. close to one there is little difference between the sine-log and sine cells. This curve is analogous to the Archimedes spiral curve.
FIG. 9 shows a single sinuous arm of the antenna of FIG. 7, defined by two curves 17,18 for the p.sup.th cell of the form ##EQU3## The two curves have the same shape as the curve of the first equation above but are rotated plus or minus .delta. degrees about the origin. The tip or outermost point of a cell occurs at the angle .alpha..sub.p +.delta. with respect to the centerline of the arm. The arm resembles a wide angle log-periodic zigzag antenna which has been distorted and curved to fit into a circular region. In contrast to a normal wire zigzag antenna the width of a sinuous arm within a cell varies with distance along the arm and the extra metal in the form of a protusion 19 and the sharp bends forms shunt capacitive loading at these points.
Further discussion of the theory of the sinuous antenna and variations may be found in the DuHamel patent.
Recently a new class of microstrip antennas has appeared which eliminates the cavity and substitutes a thin spacer and a small amount of absorber on the antenna element at the outside edge to attenuate the currents in the antenna element at the edge (see said articles by Wang and Tripp) or a loaded foam absorber starting near the outside edge but extending slightly beyond the outside edge of the antenna element to attenuate the field. The amount of absorber used is much less than for the cavity backed antenna and only a thin film of resistive material (typically a graphite loaded paint or paste) is required at the outside of the antenna element to maintain a good axial ratio and to prevent ripple in the beam pattern. The antenna element can be of the same design and construction as for the cavity loaded antenna described above and can be etched from and remain on a thin low dielectric board. The microstrip antenna is illustrated in FIG. 2. A thin spacer of low dielectric such as foamed polystyrene has been used in laboratory experiments described in recent publications. The application of the microstrip to high speed aircraft would require a different spacer than the cheap foamed polystyrene widely available from left over or surplus packing material used in the laboratory experiments. This microstrip antenna (illustrated in FIG. 2) is undergoing laboratory experiments and has not yet transitioned to aircraft deployment. This microstrip antenna of FIG. 2 employs a spacer of constant thickness. Recent published efforts also described successful application to curved surfaces.
However, all these experiments employed a constant spacer thickness. This limits the maximum bandwidth of the microstrip antenna to less than the cavity loaded design over a 300 to 400% bandwidth. The efficiency and gain degrade as the spacing increases beyond 0.2 wavelength. Operation above 12 GHz is also dependent upon the time and expense allotted to very precise construction of the antenna element near the center of a spiral or sinuous antenna element for either the cavity backed design or the microstrip design. Operation at the lowest frequencies is limited by the size or diameter of either the cavity or microstrip designs. Additionally, the microstrip is limited by the spacer thickness becoming too small at low frequencies. As the thickness decreases below 0.05 wavelength, the gain (and efficiency) are degraded. Thus, the spacer is a compromise between small spacing at high frequencies and the wider spacing a low frequencies. Gain, efficiency and impedance are effected by the compromise.
A copending U.S. patent application having the same assignee as the present application, Ser. No. 07,/578,034, now U.S. Pat. No. 5,155,493, for a "Tape Type Microstrip Patch Antenna", teaches use of tape dielectric with non-uniform thickness or dielectric properties. The same antenna is described at pages 37-40 of an unpublished technical report AFATL-TR-89-27 (available from DTIC as AD-B137 538) by Thursby et al for a "Subminiature Telemetry Antenna", from the Air Force Armament Laboratory (now part of Wright Laboratories), Eglin Air Force Base, Fla.